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  • 1
    Publication Date: 2014-04-25
    Description: A quantum computer can solve hard problems, such as prime factoring, database searching and quantum simulation, at the cost of needing to protect fragile quantum states from error. Quantum error correction provides this protection by distributing a logical state among many physical quantum bits (qubits) by means of quantum entanglement. Superconductivity is a useful phenomenon in this regard, because it allows the construction of large quantum circuits and is compatible with microfabrication. For superconducting qubits, the surface code approach to quantum computing is a natural choice for error correction, because it uses only nearest-neighbour coupling and rapidly cycled entangling gates. The gate fidelity requirements are modest: the per-step fidelity threshold is only about 99 per cent. Here we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92 per cent and a two-qubit gate fidelity of up to 99.4 per cent. This places Josephson quantum computing at the fault-tolerance threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit Greenberger-Horne-Zeilinger state using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Barends, R -- Kelly, J -- Megrant, A -- Veitia, A -- Sank, D -- Jeffrey, E -- White, T C -- Mutus, J -- Fowler, A G -- Campbell, B -- Chen, Y -- Chen, Z -- Chiaro, B -- Dunsworth, A -- Neill, C -- O'Malley, P -- Roushan, P -- Vainsencher, A -- Wenner, J -- Korotkov, A N -- Cleland, A N -- Martinis, John M -- England -- Nature. 2014 Apr 24;508(7497):500-3. doi: 10.1038/nature13171.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉1] Department of Physics, University of California, Santa Barbara, California 93106, USA [2]. ; Department of Physics, University of California, Santa Barbara, California 93106, USA. ; Department of Electrical Engineering, University of California, Riverside, California 92521, USA. ; 1] Department of Physics, University of California, Santa Barbara, California 93106, USA [2] Centre for Quantum Computation and Communication Technology, School of Physics, The University of Melbourne, Victoria 3010, Australia.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24759412" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 106 (1984), S. 817-818 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1573-5079
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The early suggestion by Lozier and Butler (Photochem. Photobiol. 17, 133–137 (1973)) that EPR Signal II arises from radicals associated with the water-splitting process in PSII has been confirmed and extended over the intervening years. Recent work has identified the Signal II radicals, $$\begin{array}{*{20}c} {\mathop D\nolimits^{\begin{array}{*{20}c} + \\ . \\ \end{array} } } \\ \end{array}$$ and $$\begin{array}{*{20}c} {\mathop Z\nolimits^{\begin{array}{*{20}c} + \\ . \\ \end{array} } } \\ \end{array}$$ , with plastosemiquinone cation species. In the experiments presented here we have used ENDOR spectroscopy and D2O/H2O exchange to characterize these paramagnets in more detail. The ENDOR matrix region, which arises from protons which interact weakly with the unpaired electron spin, is well-resolved at 4 K and at least seven resonances are apparent. A number of hyperfine couplings in the 3–8 MHz range are observed and are suggested to arise from methyl or hydroxyl protons which occur as substituents on the plastosemiquinone cation ring or from amino acid protons hydrogen-bonded to the 1,4-hydroxyl groups. Orientation selection experiments are consistent with these possibilities. D2O/H2O exchange shows that the D+/Z+ site is accessible to solvent. However, the exchange occurs slowly and is not complete even after 72 hours which suggests that the free radicals are functionally isolated from solvent water.
    Type of Medium: Electronic Resource
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